1. Field of the Invention
This invention concerns a packing seal between two parts of same geometric nature mobile in translation relatively to each other and/or between two coaxial parts mobile in rotation relatively to each other for use at high temperatures and/or under high pressure and/or speed conditions.
2. Description of the Related Art
Sealing is a function that becomes more and more important in mechanics because of the increasing use of fluids in the (hydraulic, pneumatic) drivers and in the controls, and also because of the great number and diversity of the mechanical components. The containment of a gas or a liquid is by nature difficult to obtain. Even a small leak can have multiple consequences, as to the availability of the material as well as to the operation itself and the safety. Well-known examples show that very complex units can become quite dysfunctional because of a simple leak.
Two types of sealing can be distinguished:
in the static mode, the junction must usually be removable, at least from time to time; the assembled surfaces are varied: planes, spheres, cylinders, cones, etc.;
in the dynamic mode, the junction is such that one of the two surfaces is mobile relatively to the other one, said surfaces often being of the same geometrical nature.
The packing seals for parts in translation or in rotation relatively to one another have quite a large field of applications, from the internal-combustion engine to power cylinders or turbine shafts.
In most cases, the sealing of the surfaces in relative motion must be achieved both at standstill and when these surfaces are in motion, whatever their motion speed, all the while originating as little effort as possible. There are already many solutions depending on the type of use but they are limited as to the speed, the pressure or the temperature. The packing seal must indeed ensure the sealing between the two parts with a pressure difference between its two faces. This difference in pressure generates a frictional force that, associated to the relative displacement of the two parts, releases heat. In turn, this heat, by increasing the temperature of the unit, generates frictional forces due to the expansion of the parts in contact, forces that in turn increase the temperature until a temperature equilibrium is reached that must be lower than the maximum working temperature of the materials.
The sealing during a translation movement is most often achieved by using a seal that, integral to one of the two surfaces, will come in contact with the other surface, thus subjecting it to friction. Since the sealing must also be ensured at standstill, this seal must display, in addition to its characteristics of static seal, the best possible characteristics under friction, i,e.:
minimum resistance to displacement,
negligible abrasion.
The friction generates a heat often detrimental to the operating life. Yet it must be noted that in translation the surface in contact that slides with the seal changes reciprocatively, which facilitates the thermal flow. The resistance to displacement and to abrasion is a function of many parameters, the main ones being:
the nature of the seal material,
the nature of the material of the rubbing surface,
the motion speed,
the pressure,
the fluid to be sealed,
the quality of the mobile surface (hardness, roughness, etc.).
Elastomers are often used in this type of seal despite a rather high friction coefficient and a poor thermal conductivity. With elastomers, a seal lubrication is mandatory. The main families used are:
the nitrites, especially with mineral oils;
the silicones for temperatures up to 200xc2x0 C., but they are little resistant to abrasion;
the products containing fluorine also for temperatures up to 200xc2x0 C.; they can be used with many chemicals and display good mechanical qualities;
must be added the polyurethane, very resistant to abrasion but sensitive to some liquid fluids, water in particular.
It is important to note that the material of the seal in contact with the rubbing surface heats up; consequently, its temperature may become higher than the fluid temperature. If this latter is close to the temperature limit for the use of the elastomer, a degradation could occur. This is true also of the other materials, all the more so since their thermal conductivity coefficient is low. Polyamides are fairly often used for their relatively low friction coefficient. These materials being also very poor heat conductors, they are often charged with graphite or of molybdenum bisulfide powders (that lower further the friction coefficient) and with fine metal particles (copper, lead, etc.); yet the most advantageous is the PTFE (polytetrafluoroethylene more generally known under the trademark  less than  less than  Teflon greater than  greater than ) with its very low friction coefficient (0.1 to 0.05). The PTFE allows also to work in fluids up to 250xc2x0 C. and, charged with graphite or molybdenum bisulfide, it can operate with very little lubrication if any, however under moderate pressure and speed conditions.
Metals are little used except for the rings, because of their tendency to jamming. Here too, lubrication is very important and quite mandatory. Cast iron and bronze are the two materials most commonly used. Graphite is a material fairly used for its low friction coefficient, for its good thermal transfer and for its high chemical inertia even at high temperature (400xc2x0 C. in air). Asbestos is sometimes used in association with PTFE.
In the internal-combustion engines and the compressors, sealing is achieved with split rings. The sealing rings have a rectangular cross section and their sealing function regarding gases is achieved:
by their outer face bearing on the cylinder.
by their lower face bearing on the piston groove.
So as to obtain a good sealing, the section opening needed to create the elasticity necessary to apply the ring on the cylinder must be as low as possible. The ring split leaves a passageway for burning gases along the piston, generating a pollution of the oil and heating the piston; moreover, it quickly wears out the upper part of the cylinder because of the introduction of abrasive particles. Usually, three split rings are needed to ensure a sufficient sealing, while all the manufacturers struggle to limit the number of these rings, because of their detrimental influence on the engine mechanical efficiency. It is indeed possible to estimate that each ring lowers by 104 Pa the mean effective pressure developed per engine. The non-negligible dispersal of various frictional losses in the internal-combustion engines does not hide the fact that almost half of these losses come from the piston-rings unit. In adequately optimized systems, most of these losses come from the rings alone.
What distinguishes fundamentally the seals for rotating motion from those for translation is that the mobile surface on which they effect the sealing is always essentially the same. This has important consequences as to the abrasion, heat production and release. Besides, much higher speeds can be reached. Moreover, the sealing can be achieved either directly on the shaft (radial seals), or else outside the shaft in the dynamic mode but along a direction parallel to the shaft axis (axial seals). A seal for rotating motion used under conditions of peripheral rotation speed and under a given fluid pressure can operate with other characteristics, provided that be respected the product PS (pressure . speed), equal to a constant (and expressed in Mpa. m sxe2x88x921). This is only true under well-determined conditions, especially concerning lubrication, shaft hardness and roughness etc., and for a well-defined operating life. In addition, there is a limit as to the pressure and the speed. The circular-section toric seal is suitable at low speeds (less than 0.5 m. sxe2x88x921) and at high pressures (up to 10 Mpa). A rough estimate of the PS factor is 1 for operating lives of about 200 to 500 h., and 0.1 for lives of about 5000 h., provided that there be a plentiful oil lubrication and that the shaft diameter be about 30 mm.
The lip seals are quite common. They consist of a metal frame with a L-section on which is overmolded an elastomer piece. The sealing takes place on the lip that must have a very reduced contact section with the shaft. The PS factor of the lip seals depends on the elastomer being used. It is about 0.5 for nitrile seals with a maximum peripheral speed of 15 m. sxe2x88x921 and 1.5 for seals in elastomer containing fluorine with a maximum speed of 25 m. sxe2x88x921 . Their resistance to pressure is very low, usually less than 0.05 Mpa. Yet it is possible to work them under pressures up to 1 Mpa, the speeds then not exceeding a few meters per second.
The mechanical packings display the highest PS performances . A mechanical packing is a mechanical unit ensuring the sealing in rotation by means of two plane surfaces perpendicular to the rotation axis and in relative motion vis-à-vis each other. The operation of a mechanical packing is suitable provided that some imperatives be respected. The first one is that of an appropriate lubrication. The packings allow for very high performances that also depend on the precision of their implementation and on the nature of the materials, thus actually on their cost. They can be graded according to three categories:
usual applications: PV up to 1;
good-performance applications: PV up to 20;
very high performance applications: PV up to 300.
The price ratio between the first and the last ones can increase by a 100 factor, for a given size.
For low room temperatures, the effects resulting from the expansion are minimized by using distortable materials such as elastomers or plastics, and by using whenever necessary flexible shapes such as found in lip seals or herringbone seals.
For high room temperatures, these materials are replaced with metals but then the problem resulting from the expansion becomes critical. For the rotating parts, it is possible to design so-called mechanical packings in which the faces in contact are perpendicular to the rotation axis, but for the translation there is no satisfactory solutions. For a perfect sealing, the seal must be continuous, which leads, because of the expansion, to important frictions that limit the motion speed. Higher speeds will then entail sacrificing the quality of the sealing. For turbines is used a labyrinth system that, by not bringing the parts into contact, allows for very high speeds with a constant leakage of the fluid to be drained.
For the internal-combustion engines or the piston compressors, the expansion problem is eliminated by using split rings but, there too, there is a permanent leakage with subsequent pollution problems. The sealing is then improved by increasing the number of seals, but this is at the cost of the engine efficiency.
This invention aims at providing a continuous mechanical packing that can operate at high speeds with perfect sealing, and also, since its constitutive materials can withstand higher temperatures than the elastomers, at improving the PS product and widening its application field. Moreover, this invention can provide, for specific implementations, temperature compensation means, thus allowing for work at high temperature.
According to this invention, this goal is achieved by a pressure compensation maintaining the frictional force essentially constant when the pressure increases. In addition, can be added temperature compensation means that, whatever the expansion, will hold constant the contact diameter. Said temperature compensation means will preferably be designed by coupling materials with a different thermal coefficient such as steel, titanium, aluminum alloys, with a thermal expansion coefficient varying between 8 and 24 10xe2x88x926. Besides, the contact part of the packing can benefit from an anti-abrasion surface treatment with a very wide range of procedures applicable not only to metals and metal alloys but also to polymers, composites, ceramics, etc. These could be, for instance coatings in vapor-phase, obtained either chemically CVD, or physically PVD. The PVD and CVD coatings have an excellent resistance to abrasion and the DLC (Diamond-Like Carbon) coatings an excellent friction coefficient. Such a packing will not experience jamming.
A packing seal according to this invention presents definite advantages. Because of its pressure compensation, it enables to decrease the friction forces under high pressures, thus decreasing the abrasion and improving the efficiency. Besides, to ensure the radial elasticity of the packing, the lip will be as thin and as elastic as possible. In addition to the shape of the packing characterized by a particularly judicious section, are provided implementations in materials with variable Young""s modulus, from polymers to steel alloys or ceramics.
The possibility of a thermal compensation eliminates the problem resulting from the expansion in the metal seals and provides a new tight packing seal when the only ones available so far are only operational at very low speeds and apply exclusively to mechanical packings used in rotation.
More precisely, this invention concerns a continuous mechanical packing seal with a lip, to be placed in a toroidal housing located in a first part in relative motion vis-à-vis a second part, said lip bearing on the second part so as to ensure the sealing between the packing and the second part, said packing being characterized in that it comprises such pressure compensation means that, under a pressure variation, the packing is distorted, which produces a slight motion of the lip bearing on the second part so as to hold essentially constant the bearing force of the lip on the second part.
If need be, the packing can comprise stress compensation means due to temperature. Said means affect the motion of the lip.